linux-kernel - Re: [PATCH] mm,swapops: Update check in is_pfn_swap

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Date: Sun, 7 Apr 2024 22:31:59 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Peter Xu <peterx@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Miaohe Lin <linmiaohe@...wei.com>,
	David Hildenbrand <david@...hat.com>, stable@...r.kernel.org,
	Tony Luck <tony.luck@...el.com>,
	Naoya Horiguchi <naoya.horiguchi@....com>
Subject: Re: [PATCH] mm,swapops: Update check in is_pfn_swap_entry for
 hwpoison entries

> Totally unexpected, as this commit even removed hwpoison_entry_to_pfn().
> Obviously even until now I assumed hwpoison is accounted as pfn swap entry
> but it's just missing..
> 
> Since this commit didn't really change is_pfn_swap_entry() itself, I was
> thinking maybe an older fix tag would apply, but then I noticed the old
> code indeed should work well even if hwpoison entry is missing.  For
> example, it's a grey area on whether a hwpoisoned page should be accounted
> in smaps.  So I think the Fixes tag is correct, and thanks for fixing this.
> 
> Reviewed-by: Peter Xu <peterx@...hat.com>

Thanks Peter

> Fedora stopped having DEBUG_VM for some time, but not sure about when it's
> still in the 6.1 trees.  It looks like cc stable is still reasonable from
> that regard.

Good to know, thanks for the info.

> A side note is that when I'm looking at this, I went back and see why in
> some cases we need the pfn maintained for the poisoned, then I saw the only
> user is check_hwpoisoned_entry() who wants to do fast kills in some
> contexts and that includes a double check on the pfns in a poisoned entry.
> Then afaict this path is just too rarely used and buggy.

Yes, unfortunately memory-failure code does not get exercised that much,
and so there might be subtly bugs lurking in there for quite some time.

> A few things we may need fixing, maybe someone in the loop would have time
> to have a look:
> 
> - check_hwpoisoned_entry()
>   - pte_none check is missing
>   - all the rest swap types are missing (e.g., we want to kill the proc too
>     if the page is during migration)
> - check_hwpoisoned_pmd_entry()
>   - need similar care like above (pmd_none is covered not others)

I will have a look and see what needs fixing, thanks for bringing it up.


-- 
Oscar Salvador
SUSE Labs